1. Technical Field
The present invention relates to a projector, a projection unit, and an interactive board, and in particular to a projector for ultra-short projection.
2. Related Art
In recent years, projectors each provided with a projection optical system for so-called ultra-short projection, which makes it possible to display a large screen with a short projection distance, have been put into product. By adopting such a projection optical system, it has become possible to display a large screen with a super short distance (e.g., a 100-inch screen with the projection distance of 40 cm) compared to existing front projection projectors. Such projectors are generally made usable only for super short-distance projection, and are expected to make it possible to be used for the super short-distance projection and the medium and long-distance projection of existing well-known projectors in accordance with the purposes. In, for example, JP-A-2002-6398 (Document 1), there is proposed a technology of a projector, which increases the zoom ratio using a reflective projection unit attached on the screen side with respect to the projection lens. It becomes possible to obtain an image with different zoom ratio between the state of attaching the reflective projection unit and the state of detaching it.
If the reflective projection unit according to Document 1 is applied to the super short-distance projection, there arises a necessity of substantially increasing the zoom ratio. On this occasion, the higher the zoom ratio is set, the more difficult it becomes to reduce the aberration. Further, since the configuration of disposing a plurality of curved mirrors eccentrically with respect to the optical axis (the center axis) of the lens is adopted, the case in which a minute displacement in the optical element or the like significantly affects the image increases. Therefore, an extremely accurate adjustment is required for obtaining a desired optical performance, and moreover, it becomes also difficult to reduce the aberration due to the eccentric optical system.
Further, in recent years, due to the increase in multimedia contents, a so-called interactive board used in the education field or presentations has been becoming in widespread use. The interactive board has a feature of being capable of displaying the content and at the same time allowing the user to make insertion to the content. Since the interactive board generally has a size substantially the same as those of existing blackboards or whiteboards, it results that display in relatively wide range is required. If an ordinary direct view monitor is applied to the wide-range display in the interactive board, there arises a problem in the weight, the power consumption, and the cost of the whole device.
In, for example, JP-A-2003-136892 (Document 2) and JP-A-2004-252345 (Document 3), there is proposed a technology of an interactive board which reflects the light projected from a projector with a planar mirror to thereby input it into a transmissive screen. In JP-A-2009-83277 (Document 4) there is proposed a technology of an interactive board provided with a projector for super short-distance projection. By the enlarged projection using a projector, display in a wide range is made possible. By using a projector, reduction in weight, power consumption, and cost becomes possible. It should be noted that in the technologies of Documents 2 and 3, there arises a problem that the size in the depth direction grows to thereby degrade facility of installation since the large planar mirror disposed at an angle with the transmissive screen. The projector in the technology of Document 4 is only applied to the super short-distance projection, and has a problem that it is low in convenience because of the limited usage.